1. Field of the Invention
The present invention relates to integrated circuits, and more specifically to a method and apparatus for reducing electro magnetic interference (EMI) in integrated circuits which operate on both analog and digital signals (“mixed signals”).
2. Related Art
Integrated circuits often operate on both analog and digital signals. A digital signal is generally associated with a value depending on the specific one of the several possible discrete signal levels. For example, a voltage level of approximately +5 Volts may be interpreted as a logical one and 0 Volts (or −5 volts) may be interpreted as a logical zero.
On the other hand, an analog signal is generally characterized by a continuous signal level. As an illustration, an analog signal may have any voltage level between a maximum and minimum values at a specific point of time. Integrated circuits operating on both analog and digital signals are commonly referred to as mixed signal circuits.
Mixed signal circuits are used in several areas such as communications (transmission and reception of signals), industrial applications (e.g., motor control, power control), consumer devices (e.g., camera, watch). For example, a mixed signal circuit may receive an analog signal representing voice and convert to corresponding digital samples. The digital samples may then be processed in a desired manner in a digital domain. In the alternative, digital signals may be converted to an analog signal (e.g., sound) by mixed signal circuits.
One problem often encountered in mixed signal circuits is Electro Magnetic Interference (EMI). EMI generally refers to undesirable noise generated when a signal is transmitted on/through an impedance. The EMI caused by one (“first signal”) of the analog and digital signals on the other (“second signal”) may be unacceptably high when the first signal is substantially stronger than the second signal.
For example, a low voltage analog signal may be received on a pin of a mixed signal circuit, and digital samples may be generated based on the analog signal. The digital samples may need to be provided with large current, for example, to drive higher loads (or low impedance load). The EMI resulting from the large current may introduce unacceptably high noise levels into the input analog signals.
In one prior approach, the undesirable noise is reduced (or eliminated) by operating a mixed signal circuit in differential mode, in which a signal level is determined based on the difference of signal levels on two paths/points. Due to the determination of the difference, the noise in the two paths/points may get canceled. One problem with such an approach is that more pins and/or extra circuitry may be required. The resulting extra circuitry and space consumption may be undesirable.
What is therefore required is a method and apparatus for reducing electro magnetic interference (EMI) in integrated circuits which operate on both analog and digital signals.